In real life, it's a phenotype occasionally associated with a number of unrelated genetic defects. It can also be the result of injury to the scalp. There's a scientific term for this, it's called poliosis.

A subtrope of Multicolored Hair. Actual skunks are, of course, assumed to have this style..

The first time Guts uses the Berserker's Armor in Berserk, he gets a white streak in his hair.

Death the Kid from Soul Eater has multiple white streaks in his black hair. Interestingly enough (and occasionally Lampshaded), the streaks are aligned horizontally across his hair, rather than running along vertically. Specifically stated to be a result of his death-god father mystically coloring or recoloring his hair for the Rule of Cool. Unfortunately for Kid, he has OCD and is absolutely obsessed with symmetry; the stripes are a constant source of chagrin for him; on the plus side, the manga has revealed that, eventually, Kid will have three complete stripes in his hair. It's a grown-up death god thing.

In Black Jack, Dr. Black Jack sports a large white streak through his hair thanks to an accident that effectively blew half of his face off. He got better.

The Dirty Pair Flash version of Kei has a blonde forelock and is otherwise a redhead.

In the Monster Collection manga, summoners often have hair like this. Apparently, it happens when the summoned creature dies, because of Synchronization.

Marco from Eyeshield 21 has a lightning-shaped skunk stripe running through his slicked back brown hair.

Takuto from Argento Soma gets white streaks on his left side, not to mention facial scars on his cheeks (badass) and eyelids (a beautiful gray shade), courtesy of an experiment gone horribly wrong (trying to bring to life a creature pieced together from various destroyed aliens that have been plaguing the planet may not be a good idea). He keeps these scars despite the availability of plastic surgery, because of the guilt he had over his inability to save his girlfriend from said creature, so it seemed. Turns out, said creature, later called Frank, was trying to save her from the crumbling building, but he was too late.

The jury's still out on how Rogue of the X-Men got hers (she's even called "skunk hair" by some punks in an episode of X-Men Evolution!). Some sources say it came with the very first use of her powers. Some say it was from the absorption of her more conventional superpowers. A letter column from Marvel once stated it was dyed in. That was printed in the 1980s and may no longer be valid; then again, the white will go down the middle of her hair one month, then be just a few bangs the next... Meanwhile, the character's more recent solo title depicts her as simply being born with the white streak.

The first of the three movies pins it on forced overuse of Magneto's power after she was hooked to the life-draining mutancy machine. She admits that she likes how it looks, but in the second film, Magneto and Mystique taunt her by saying "We love what you did with your hair".

And according to quite a bit of questionable fanart, said "stripe" extends to more than just what's on her head...

Nate Grey, another X-Man, also has white in his otherwise light brown hair.

Nocturne also has a streak of white hair. This seems to be a stylistic choice, however.

The Red She-Hulk inverts this, with black hair with two stripes of red hair running through it, enhancing her similarity to a demon.

The eponymous character of XIII has a grey streak on his left temple where a bullet grazed him pre-amnesia.

In the DC Universe, resurrection in a Lazarus Pit always causes a white streak, though some people (like the Riddler) hide it with dye. As a result, Ra's Al Ghul has two thin streaks of gray in his black hair—it's hinted that he keeps them because he likes how they looks. This is kept in all his incarnations. It even became a sort of Foreshadowing in a Batman Beyond episode in which "Talia" lets Bruce try the Lazarus pits, and she remarks that Young!Old!Bruce's similar greying pattern is "Quite dignified"; Bruce also gets the stripe in Superman Batman Generations when Ra's convinces him to use the Pit.

Older versons of Hal Jordan have white/grey streaks at their temples. The mainstream version of the character also had them for a while. Originally, it was supposed to be age, later retconned as a premature stress induced greying brought on by Parallax (Infection by a being of pure fear turning his hair white) and fixed when he was resurrected.

Repeated, complete with a true Skunk Stripe along the middle, when Kyle Rayner becomes Parallax.

Songbird, in the Thunderbolts, has red (or pink) streaks in white hair.

Both Jason Blood and the original Jim Corrigan had/have a white Skunk Stripe on dark auburn hair. In both cases, it's from exposure to Powers Greater Than Mortal Man—but at opposite ends of the spectrum (Blood is bonded to the Demon Etrigan, while Corrigan was the host for The Spectre). AFAIK, they never showed up in the same story.

The original John Gaunt incarnation of Grimjack had this, which seemed unrelated to his trademark scar. A clone body he inhabits later also has it, so it seems to be genetic.

Knives Chau of Scott Pilgrim dyes a pink one into her bangs, but during her encounter with the Clash at Demonhead, Lynette punches her so hard that the stripe is literally knocked out of her hair. It's blue in the movie. In both cases, Knives is trying to emulate Ramona's current look to try and catch Scott's attention again.

Inverted and subverted in Bolt, where the white-coated dog believes the bolt-shaped patch of black fur on his side is proof he was given superpowers. Once he's away from the studio for a while, the black dye fades and reveals that this mark, like Bolt's "powers", was all a lie.

The main character in the French-Canadian film C.R.A.Z.Y., Zac (Marc-André Grondin), has a white streak in the back, caused by having been dropped on his head as a newborn. His mother convinces him as a child that it's a sign he has the ability to heal others through prayer. Also an interesting mirror to his friend Michelle, who has light hair with a dark streak.

Death's granddaughter, Susan Sto Helit, inverts this trope: her hair is naturally white with a black streak (as well as expressive). In The Movie of Hogfather, her hair is platinum blonde instead of pure white, but she retains the black streak.

In Stephen King's The Stand, the character of Nadine starts with black hair, gains a white stripe, then ends up all-white as a result of her personal proximity to Randall Flagg.

In David Eddings' Belgariad and Malloreon series, Polgara has a white lock. She got it when her father, the sorcerer Belgarath, first laid his hand upon her after she was born. It resists cutting and dyeing, which leads to some interesting dilemmas when they're trying to hide from their enemies. A similar white mark can be found on all the sorcerers in Garion's line, but on different parts of their bodies.

In Memory Sorrow and Thorn by Tad Williams, Simon gains a scar and white streak in his hair after being burned by dragon's blood. From then on, he's often referred to as Simon Snowlock.

The title character in Tailchasers Song, also by Williams, has this, except, being a cat, it's through his fur.

In The Rowan and its sequels by Anne McCaffrey, the titular character has white-blonde hair, but her descendants tend to have dark hair with a white streak. Some are blonde, still with the white streak.

A good portion of Horror Campfire stories has a character end up with white streaks—or all white-hair—after being frightened by some sort of supernatural encounter.

At the end of Robin Hobb's Farseer trilogy, Fitz has developed a white streak in his hair due to a head wound. In the Tawny Man trilogy, set fifteen years later, he's started using the alias Tom Badgerlock, claiming he was born with the white streak. It eventually disappeared when he was magically healed in the Tawny Man books.

Eldan in Mercedes Lackey's By the Sword has dark hair with white streaks at his temples.

In the Eighty Seventh Precinct novels by Ed McBain, Detective Cotton Hawes has a white streak in his otherwise red hair as a result of his hair growing back over a knife scar.

Kerish in Geraldine Harris' The Seven Citadels tetralogy has black hair with a broad silver stripe. As the series progresses, the stripe widens until he has all-silver hair.

Erast Fandorin's temples become gray following the shock of witnessing his first wife death in a terrorist attack (originally aimed at him!) at the end of the first novel. His gray temples later become his characteristic mark, which is used by an impostor in a later novel to pass as Fandorin.

The protagonist of the Larry Niven story "Relic of Empire" lost his fortune and began to get gray hair before he could once again afford anti-aging drugs. After rejuvenating, he discovered that the gray streaks gave him a distinguished look that attracted the ladies and started dyeing them back in.

In The Magic Goes Away, one sorceress has a white streak in her hair because of the difficulty of maintaining her youth without the mana to do it. The streak increases and decreases opposite to the amount of mana in the environment.

Richard Sharpe is described as having gotten one by the time of the last chronological book, Sharpe's Devil.

One of the families in Edward Rutherfurd's London has this trope as one of its hereditary genetic quirks.

In the Black Jewels trilogy, a witch named Luthvian is described as having a white streak in her hair that is "a memory of her father's fists."

In the series finale, "All Good Things", future Data has a (possibly bleached) stripe of white hair that his housekeeper absolutely despises. (In another episode, he states that he can manipulate the length of his hair at will, but it's unclear if he can change his hair color on his own.)

The Rutians, aliens of the week: the males all have black hair with a white stripe, while the females all have red hair with a black stripe.

Also, Vida, the Pink Ranger in Mystic Force. She sometimes dyes hers different colors, until pink (a color which, ironically, she used to dislike) gets to her and she uses it on her entire hair at the finale.

Vladimir Von Helson of Gaia Online fame has two thin white streaks in his hair...and a big Skunk Stripe in his goatee. This is now immortalized as the bonus pose of the doll item in his image. Too bad the same can't be said of the original...

Captain Cross from Prototype has a white stripe in his hair. Considering the big honking scar that runs across his face, this could probably be explained by trauma to the scalp as well. (The prequel comic by Wildstorm offers a probable explanation: bitten in the head by zombie wolf.)

The anthropomorphic raccoon hero of Tales of the Questor has a white cow-lick where he was touched by something divine. According to the author, not God, but a mythic creature of apparent divine connection or insight, potentially an Angel Unaware.

Main character Steve has completely black hair save for two blonde bangs. Initially, you'd think these were dyed in. Then it turns out that his sister has blonde hair that turns to black halfway down, his mother has black hair with a blonde streak, and all three of his children invariably have black hair with a blonde streak (at least, until Mary dyes her hair bright green after the Timeskip).

In Gunnerkrigg Court, inside Dr. Disaster's space battle simulator, all the students gain a stripe of differently-colored hair. Redheaded Hero Antimony gets a lock of black; black-haired Kat gets a lock of red. Aside from the chapter title page, the simulator is depicted Deliberately Monochrome, so it's unclear exactly what colors the other students got. Tom Siddell originallyplanned to make these Skunk Stripes permanent.

Mistake in Dandy and Company has a white stripe running through his hair and all the way down his face. This was seen in an arc that takes place in the future and eventually shown in the present-day when one of his Homemade Inventions goes berserk.

Hanna Is Not a Boys Name's unnamed zombie narrator has black hair, with each temple having this little white tuft like an angel wing growing out of it. He didn't have them when he was alive, but it's not clear where they came from or whether they're typical for zombies.

Apparently as a side effect of her sex change, Jet Dream's Harmony has a scarlet red streak in her otherwise black hair.

Chelsea of Clan of the Cats acquires a Skunk Streak after one of the early story arcs where she becomes accustomed to magic and being a werepanther.

The Secret Saturdays' Zak Saturday has a star-shaped patch of white hair. This could be some weird inheritance from his mother, who has white hair, or it could be a sign of his special cryptid powers. His father seems to display the Skunk Stripe as well.

When Danny gave up his ghost powers, a white streak was left. Compare to Vlad, who was born with the skunk stripe.

Danny's Bumbling Dad, Jack, apparently was part of a band called the "Skunk Punks", where all players had the stripe. Unfortunately, it invited the headline "Skunk Punks stink", which Jack admits was inevitable.

Butch in Recess, who reportedly got it because the sight of a teenage couple kissing was just that scary. He actually was seen with plain brown hair in at least one episode before that.

Isaac Sumdac in Transformers Animated. We saw it in flashbacks, suggesting that it was genetic, but then we saw him get it from the shock of touching the protoform that would later become his daughter, Sari.

The titular character of Freakazoid!! has a lightning shaped streak in his hair.

Animators in general like doing this with Count Dracula, whether he's appearing as a cameo on shows like Scooby Doo or as a series regular as on Drak Pack. Whether his Skunk Stripe is down the middle or at the temples depends on how old he's made to appear.

The undead Captain Skunkbeard in Scooby Doo Pirates Ahoy! was named for the white strip in his beard. Subverted because it's only a costume, surprise surprise.

Tony Spilotro, the mobster played by Joe Pesci in Casino, really did have a gray stripe in his otherwise dark brown hair.

Both David and Ed Miliband, British politician brothers, have a small patch in the same spot on their heads and are often caricatured in the media for it. As did their late father, Ralph, a British political academic.

Jay Leno. Though, by now, it's an inversion: there's a small dark patch with everything else being gray. Leno actually made a skit making fun of that dark patch. He was sitting in his office when that Zoo lady came in with a squid and it squirted him in the face.

Susan Sontag was famous for her dark hair with a thick white stripe.

Dave Vanian of punk band The Damned had Depp's Sweeney hair thirty years before Depp.

Samuel Beckett had a black streak through his white hair.

John Henson, who was regularly called "Skunk-Boy" when he hosted Talk Soup.

Janet Napolitano, US Secretary of Homeland Security. Combine the hair and the title with the fact that she has climbed Mt. Everest, and she begins to sound like a character out of a video game. Unfortunately, she's a rather plump and matronly looking middle-aged woman.

Stacy London of the US version of What Not To Wear has a fairly subtle grey forelock which comes and goes on the show—depending (as she's said) on how they style and part her hair—but is quite prominent in some of her more recent ad appearances.

Arlene Dickinson, CEO of Canada's Venture Communications, is a celebrity businesswoman and regular on CBC's Dragons' Den. She has crimped red hair with a prominent white streak.

Indira Gandhi—at least, during the time when she was Prime Minister of India—had a white streak in her dark hair.

This is a feature of Waardenburg Syndrome, along with odd facial appearance and occasional deafness.

Joe Perry from Aerosmith, as of 2009, has a white stripe in the front of his mostly dark brown/black hair.